System, method and apparatus for polarization control

ABSTRACT

A polarization control device includes a first wave plate having a first surface profile and a second wave plate having a second surface profile complementary to the first surface profile. The optical axis of the first wave plate is orthogonal to the optical axis of the second wave plate. The first wave plate and the second wave plate are positioned to align the first surface profile with the second surface profile and maintain a constant thickness across the polarization control device. The first wave plate and the second wave plate may control polarization rotation as a continuous function of transverse position across a pupil plane of an optical system. The first wave plate and the second wave plate are separated by a sufficiently small distance so as to limit wave front distortion below a selected level.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims benefit of the earliestavailable effective filing date from the following applications. Thepresent application constitutes a divisional patent application ofUnited States Patent Application entitled SYSTEM, METHOD AND APPARATUSFOR POLARIZATION CONTROL, naming Ivan Maleev and Donald Pettibone asinventors, filed Jun. 4, 2014, application Ser. No. 14/296,425, which isa regular (non-provisional) patent application of United StatesProvisional Patent Application entitled POLARIZATION CORRECTION DEVICEFOR HIGH NA UV OPTICAL SYSTEMS, naming IVAN MALEEV as inventor, filedJun. 6, 2013, Application Ser. No. 61/831,950. U.S. patent applicationSer. No. 14/296,425 and U.S. Provisional Patent Application No.61/831,950 are incorporated by reference herein in their entirety.

TECHNICAL FIELD

The present invention generally relates to polarization control in anoptical system. In particular, the present invention relates topolarization control in a high NA inspection system.

BACKGROUND

As the demand for integrated circuits having ever-smaller devicefeatures continues to increase, the need for improved substrateinspection systems continues to grow. One aspect of inspection tooloperation includes the control of polarization within the opticalpathway of the inspection system implemented in order to analyze variousaspects of defect, particle or surface features of a given sample, suchas a semiconductor wafer. Traditional polarization control devicesinclude electronically-controlled spatial light modulator devices.Typical spatial light modulators utilize liquid crystals, whichcurrently are not suitable with UV light (λ<300 nm). In addition,previous spatial control of polarization within an optical pathwayprovides for discrete changes in polarization retardation. Such systemsmay result in a significant amount of stray light production due toscattering from pixel boundaries.

It is further noted that any polarization control mechanism must competewith other optical requirements, such as transmission, field of view,high numerical aperture, and control of aberrations. For example, anall-refractive element based optical system may result in a high numberof such elements, which are all necessary to control different types ofaberrations, which results in reduced system transmission, which, inturn, leads to higher cost. In addition, other optical designs, such asa parabolic mirror or simple Swartzschild objective, have intrinsicoptical limitations, which, for example, may lead to various degrees ofpolarization aberration.

Another aspect of inspection tool operation includes the control ofpoint spread function (PSF) at an imaging detector. Typical opticalsystems minimize PSF size by increasing numerical aperture (NA) andreducing aberrations in an effort to obtain a PSF as close to thediffraction limit as possible. However, in applications related todetecting small surface defects via light scattering techniques, themaximization of signal-to-noise ratio (SNR). One approach typically usedto maximize SNR is to minimize unwanted noise originating from lightscattered by a surface, while maintaining (or increasing) light from adefect of interest. Typically, this can accomplished via control of thetransmitted polarization, and by mechanically limiting an open aperture.Transmission and suppression of polarizations of interest over a pupilplane may be controlled with single or multiple mechanical polarizationelements in a pupil plane, with transmitted light having varyingpolarization across the pupil plane. However, such measures result indegrading PSF away from the diffraction limit. Therefore, it would bedesirable to provide a system and method for curing defects such asthose of the identified above.

SUMMARY OF THE INVENTION

A polarization control apparatus is disclosed, in accordance with anillustrative embodiment of the present invention. In one illustrativeembodiment, the polarization control apparatus may include a first waveplate having a first surface profile and a second wave plate having asecond surface profile complementary to the first surface profile. Inone embodiment, the optical axis of the first wave plate issubstantially orthogonal to the optical axis of the second wave plate.In another embodiment, the first wave plate and the second wave plateare positioned so as to substantially align the first surface profilewith the second surface profile and maintain a constant thickness of anassembly of the first wave plate and second wave plate. In anotherembodiment, the first wave plate and the second wave plate areconfigured to control polarization rotation as a continuous function oftransverse position across a pupil plane of an optical system. Inanother embodiment, the first profile of the first wave plate isseparated from the second profile of the second wave plate by a selecteddistance so as to limit wave front distortion of illumination passingthrough the first wave plate and second wave plate below a selectedlevel.

In another illustrative embodiment, polarization control apparatus mayinclude a single wave plate having a surface profile configured tocontrol polarization rotation as a continuous function of transverseposition across a pupil plane of an optical system.

An optical system for controlling polarization is disclosed, inaccordance with an illustrative embodiment of the present invention. Inone illustrative embodiment, the optical system may include anillumination source configured to illuminate a surface of a sample, aset of collection optics configured to collect illumination from thesurface of a sample and a polarization control device positionedsubstantially at a pupil plane of the optical system. In one embodiment,the polarization control device may include a first wave plate having afirst surface profile and a second wave plate having a second surfaceprofile complementary to the first surface profile. In one embodiment,the optical axis of the first wave plate is substantially orthogonal tothe optical axis of the second wave plate. In another embodiment, theoptical axis of the first wave plate and the optical axis of the secondwave plate are substantially orthogonal to a direction of illuminationpropagation through the polarization control device. In anotherembodiment, the first wave plate and the second wave plate arepositioned so as to substantially align the first surface profile withthe second surface profile and maintain a constant thickness of anassembly of the first wave plate and second wave plate. In anotherembodiment, the first wave plate and the second wave plate areconfigured to control polarization rotation as a function of transverseposition in a pupil plane of the optical system. In another embodiment,the optical system includes a linear polarizer configured to receiveillumination transmitted through the polarization control device and asensor configured to detect illumination transmitted through the linearpolarizer.

An optical system for controlling point spread function is disclosed, inaccordance with an illustrative embodiment of the present invention. Inone illustrative embodiment, the optical system includes an illuminationsource configured to illuminate a surface of a sample, a set ofcollection optics configured to collect illumination from the surface ofa sample, a detector including a plurality of pixels and a point spreadfunction control device positioned substantially at a pupil plane of theoptical system. In one embodiment, the point spread function controldevices may include a first wave plate having a first surface profileand a second wave plate having a second surface profile complementary tothe first surface profile. In another embodiment, the first wave plateand the second wave plate are positioned so as to substantially alignthe first surface profile with the second surface profile and maintain aconstant thickness of an assembly of the first wave plate and secondwave plate. In another embodiment, the first wave plate and the secondwave plate are configured to control at least one of polarizationrotation and degree of coherence as a continuous function of transverseposition across the pupil plane in order to modify the point spreadfunction of illumination in order to enhance an amount of energyreceived by one or more pixels of the detector.

In another illustrative embodiment, the optical system for controllingpoint spread function may include an illumination source configured toilluminate a surface of a sample, a set of collection optics configuredto collect illumination from the surface of a sample, a detectorincluding a plurality of pixels and a point spread function controldevice positioned substantially at a pupil plane of the optical system.In one embodiment, the point spread function control devices may includea first wave plate having a first surface profile and a second waveplate having a second surface profile complementary to the first surfaceprofile. In one embodiment, the first wave plate and the second waveplate are positioned so as to substantially align the first surfaceprofile with the second surface profile and maintain a constantthickness of an assembly of the first wave plate and second wave plate.In one embodiment, the first wave plate and the second wave plate areconfigured to impart an optical delay as a continuous function oftransverse position across a pupil plane of an optical system in orderto modify the point spread function of illumination in order to enhancean amount of energy received by one or more pixels of the detector.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the invention as claimed. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1A illustrates a block diagram view of a system implementing apolarization control device, in accordance with one embodiment of thepresent invention.

FIG. 1B illustrates an exploded schematic view of the polarizationcontrol device, in accordance with one embodiment of the presentinvention.

FIG. 1C illustrates an assembled schematic view of the polarizationcontrol device, in accordance with one embodiment of the presentinvention.

FIG. 1D illustrates a flow chart depicting a method of making thepolarization control device, in accordance with one embodiment of thepresent invention.

FIG. 1E illustrates a graph including a conceptual depiction of thedirection of polarization for both a signal and the surface background,in accordance with one embodiment of the present invention.

FIG. 1F illustrates a conceptual illustration of the conversion ofelliptically polarized background signal into linear polarized light, inaccordance with one embodiment of the present invention.

FIG. 2 illustrates a simplified schematic view of an inspection systemwith a parabolic collector implementing a polarization control device,in accordance with one embodiment of the present invention.

FIG. 3 illustrates a simplified schematic view of an inspection systemwith a Schwarzchild objective implementing a polarization controldevice, in accordance with one embodiment of the present invention.

FIG. 4 illustrates a schematic view of a single-plate polarizationcontrol device, in accordance with one embodiment of the presentinvention.

FIG. 5A illustrates a simplified schematic view of an optical systemimplementing a PSF control device, in accordance with one embodiment ofthe present invention.

FIG. 5B illustrates a conceptual view of a point spread functionoverlapping multiple pixels of a detector array, in accordance with oneembodiment of the present invention.

FIG. 5C illustrates a conceptual view of split point spread functionsfrom a single illumination source matched to the size of the pixels of adetector array, in accordance with one embodiment of the presentinvention.

FIG. 5D illustrates a schematic view of a point spread function controldevice, in accordance with one embodiment of the present invention.

FIG. 5E illustrates a schematic view of a point spread function controldevice, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the invention as claimed. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention. Reference will now be made in detail to the subjectmatter disclosed, which is illustrated in the accompanying drawings.

Referring generally to FIGS. 1A through 5E, an apparatus and system forpolarization and/or point spread function control in an optical systemare described, in accordance with one or more embodiments of the presentdisclosure.

Embodiments of the present disclosure are directed to one or morepolarization control devices suitable for controlling polarization as acontinuous function across the pupil plane of an optical inspectionsystem, such as, but not limited to, a high numerical aperture (high-NA)ultraviolet (UV) inspection system. The polarization control device ofthe present disclosure may allow for the correction of aberrationscreated by one or more optical elements of the particular opticalsystem. In addition, the polarization control device may further providefor the enhancement of the signal-to-noise ratio via the suppression ofan unwanted signal (e.g., surface haze) relative to a desired signal(e.g., scattering signal from particle). Embodiments of the polarizationcontrol device of the present disclosure accomplish such spatiallycontinuous polarization control across the pupil plane of an opticalsystem via the implementation of two phase plates with complementarysurfaces, described further herein. The applications of such a device ina UV inspection system, for example, include the ability to relax therequirements of other, more complex, optical components. For instance,the polarization control outlined in the present disclosure may allowfor the use of catadioptric systems or other optical designs, whichsubstantially distort input polarization distributions over NA.

Additional embodiments of the present disclosure are directed to one ormore point spread function control devices suitable for controlling thepoint spread function of illumination impinging on a detector array of adetector. The point spread function control devices described herein arecapable of modifying the point spread function of an optical systemthrough the modulation of phase and/or degree of coherence over thepupil plane of an optical system. This capability may serve to enhanceor maximize the energy received by a single pixel in a detector array.

FIG. 1A illustrates a conceptual block diagram view of a system 100having polarization control capabilities, in accordance with one or moreembodiments of the present disclosure.

In one embodiment, the system 100 includes an illumination source 101.In one embodiment, the illumination source 101 is configured toilluminate the surface of the sample 114 with illumination 113. Thesample 114 may include any substrate, wafer or specimen known in theart, such as a semiconductor wafer. The illumination source 101 mayinclude any illumination source known in the art of sample inspection.For example, the illumination source 101 may include, but is not limitedto, a narrow band light source (e.g., one or more lasers). By way ofanother example, the illumination source 101 may include, but is notlimited to, a broad band light source (e.g., laser-sustained plasmalight source, discharge lamp and the like). Further, the illuminationsource 101 may illuminate the surface of the sample 114 withillumination 113 of any spectral range known in the art of sampleinspection. For example, the illumination source 101 may be configuredto illuminate a portion of the sample 114 with one or more ofultraviolet or visible light. In another embodiment, the system 100includes a set of illumination optics (not shown) for directing and/orfocusing illumination 113 emitted by the illumination source 101 onto aselected portion of the surface of the sample 114. For example, the setof illumination optics may include, but is not limited to, one or morelenses, one or more mirrors, one or more beam splitters, one or morefilters and the like.

It is noted herein that the optical system 100 may include any opticalinspection system known in the art. In one embodiment, the inspectionsystem may include, but is not limited to, a reflective-based inspectionsystem (e.g., system 200, system 300 and like systems). Examples ofreflective-based systems are described further herein. In anotherembodiment, the optical system 100 may include a refractive-basedinspection system (e.g., system 500). In one embodiment, the opticalsystem 100 may include, but is not limited to, a high-NA inspectionsystem. In another embodiment, the optical system 100 may include ahigh-NA ultraviolet inspection system.

In another embodiment, the system 100 includes a set of collectionoptics 106 a, 106 b. In one embodiment, the set of collection optics 106a, 106 b are configured to collect illumination from the surface (e.g.,light scattered from one or more defects, light reflected from samplesurface and the like) of the sample 114. The collection optics 106 a,106 b may include any set of collection optics known in the art ofsample inspection. For example, the collection optics 106 a, 106 b mayinclude, but are not limited to, a Schwarzchild objective with tubelens, a Schwarzchild objective with an afocal lens, one or moreparabolic collectors and the like. In another embodiment, the collectionoptics 106 a, 106 b may include a set of refractive optics. For example,the collection optics 106 a, 106 b may include, but are not limited to,a catadioptric objective. It is noted herein that particularimplementations of inspection architecture are described in greaterdetail further herein.

In one embodiment, the optical system 100 includes a polarizationcontrol device 102, in accordance with one embodiment of the presentdisclosure. In one embodiment, the polarization control device 102 ispositioned at the pupil plane 103 of the optical system 100. As noted ingreater detail further herein, the polarization control device 102 isconstructed so as to control polarization rotation continuously as afunction of position across the pupil plane 103 of the optical system100. In one embodiment, such polarization control across the pupil plane103 may be used to correct polarization aberrations in the opticalsystem 100. In another embodiment, the polarization control across thepupil plane 103 may be used to alter and/or suppress an undesirablepolarization distribution from the sample 114.

In another embodiment, the optical system 100 includes a linearpolarizer 104. In one embodiment, the linear polarizer 104 may also besubstantially positioned at the pupil plane 103. In one embodiment, thelinear polarizer 104 may be used to analyze the polarization of lightfollowing the polarization operations imparted to the light (as afunction of x,y position) by the polarization control device 102.

In another embodiment, the system 100 includes a sensor 110. In oneembodiment, the sensor 110 is configured to detect illumination from thesurface of the sample 114 upon transmission of the illumination throughat least the polarization control device 102 and the polarizer 104. Thesensor 110 may include any optical sensor known in the art of sampleinspection. For example, the sensor 110 may include, but is not limitedto, one or more CCD detectors. By way of another example, the sensor 110may include, but is not limited to, one or more TDI-CCD detectors.

FIGS. 1B and 1C illustrate simplified schematic views of the dual-platepolarization control device 102, in accordance with one or moreembodiments of the present disclosure. FIG. 1B illustrates a simplifiedexploded schematic view of the polarization control device 102, inaccordance with one or more embodiments of the present invention. FIG.1C illustrates a simplified assembled schematic view of the polarizationcontrol device 102, in accordance with one or more embodiments of thepresent invention.

In one embodiment, the polarization control device 102 is placed in thepupil plane 103 of the optical system 100, as shown in FIG. 1A. In oneembodiment, the optical axis 121 of the first plate 116 is orientedsubstantially orthogonal to the optical axis 123 of the second plate118. In another embodiment, both the optical axis 121 of the first plate116 and the optical axis 123 of the second plate 118 are orientedsubstantially orthogonal to the direction 125 of illuminationtransmission through the polarization control device 102. It is notedherein that the orthogonal alignment of the optical axes 121, 123 of thefirst and second plates 116, 118 provides for birefringence in thecombined plate polarization control device 102.

In one embodiment, the first wave plate 116 and the second wave plate118 are configured to continuously control polarization rotation as afunction of transverse position in the pupil plane 103 of an opticalsystem 100.

In one embodiment, the first wave plate 116 and the second wave plate118 control polarization rotation in a continuous manner (i.e.,non-discrete) across the pupil plane 103 of an optical system bycontrolling the level of retardance continuously across the pupil plane103. In this regard, the first wave plate 116 and the second wave plate118 serve to continuously control the phase delay between polarizationstates (e.g., s-polarization and p-polarization) of light passingthrough the polarization control device 102, with the amount of thedelay being a function of the position of the light incident on thepolarization control device 102. As shown in FIGS. 1B and 1C, the phasedelay is controlled as a function of position across the pupil plane bycontrolling the thickness of one or more individual plates 116, 118 as afunction of transverse position (e.g., x, y position) across the pupilplane 103.

By controlling the amount of imparted polarization rotation as afunction of position across the pupil plane 103, the polarizationcontrol device 102 may form a continuous ‘polarization map’ across thepupil plane 103. It is noted herein that the utilization of continuouspolarization control over NA is particularly advantageous as it avoidsscattering from discrete polarizing elements, which is commonlyencountered in discrete polarization control schemes.

In one embodiment, the polarization control device 102 includes a firstwave plate 116 having a first surface profile 117. In anotherembodiment, the polarization control device 102 includes a second waveplate 118 having a second surface profile 119. In one embodiment, theprofile 119 of the second wave plate 118 is complementary to the profile117 of the first wave plate 116. The complementary surface profiles 117,119 of the first wave plate 116 and the second wave plate 118 allow forclose alignment of the first wave plate 116 and second wave plate 118.In this regard, the complementary surface profiles 117, 119 of the firstwave plate 116 and the second wave plate 118 allow the wave plates 116,118 to be aligned in a manner such that a combined assembly of the firstwave plate 116 and second wave plate 118 has a substantially constantthickness d, as shown in FIG. 1C. It is noted herein that thecombination of the first wave plate 116 and the second wave plate 118having a substantially constant thickness (and spacing between plates)aids in limiting distortion(s) of the wave front passing through thepolarization control device 102.

In another embodiment, upon forming the complementary surfaces 117, 119,the first plate 116 and second plate 118 may be positioned proximate toeach other, as shown in FIG. 1C. In one embodiment, the surface profile117 of the first wave plate 116 is separated from the surface profile119 of the second wave plate 118 by a selected nominal distance that issufficiently small so as to limit wave front distortion of illuminationpassing through the first wave plate 116 and second wave plate 118(e.g., limit below a selected level). For example, the surface profile117 of the first plate 116 an the surface profile 119 of the secondplate 118 may be positioned sufficiently close to teach other so thatthe local tilt of the surfaces 117, 119 does result in local wave frontshift and corresponding distortions.

In another embodiment, the surface 117 of the first plate 116 and thesurface 119 of the second plate 118 may be operably coupled to eachother. In one embodiment, the surface 117 of the first plate 116 and thesurface 119 of the second plate 118 may be affixed (e.g., affixed usingglue, epoxy and the like) to one another in order to minimize light lossin the interface 120. In another embodiment, an index-matching fluid maybe disposed in the interface 120 between the surface 117 of the firstplate 116 and the surface 119 of the second plate 118. In anotherembodiment, the surface 117 of the first plate 116 and the surface 119of the second plate 118 may be mechanically coupled (e.g., clamp) toeach other in order to minimize light loss in the interface 120.

It is noted herein that the local shift of the wave front is defined bythe local surface tilt at the interface between two plates 116,118. Forexample, in the case of magnesium fluoride (MgF₂) where thecharacteristic distance for the change of phase delay by 2π inx-direction is 10 mm with a surface profile described by a sinefunction, the surface may display a period of 20 mm and an amplitude of5.244 μm (2π*1.6694). This then corresponds to a profile function z(x,y)(measured in μm) as below:

${z\left( {x,y} \right)} = {5.244*{\sin\left( \frac{2\pi\; x}{20\text{,}000} \right)}}$

Further, the derivative of the profile function is given by:

${z^{\prime}\left( {x,y} \right)} = {5.244*\frac{\pi}{10\text{,}000}{\cos\left( \frac{\pi\; x}{10\text{,}000} \right)}}$

As such, the maximum local tilt in this example is:

$T = {5.244*\frac{\pi}{10\text{,}000}}$

The above tilt corresponds approximately to 1.65 mrad. With an interface120 index of refraction of n, and beam tilt values defined by Snell'slaw, for a normal incidence beam at the exit of first plate 116, theangle G of the output beam with respect to the normal is given by sin(G)=sin(T)*n_(e)/n. For instance, in the case of an air interface 120, Gis approximately 2.3 mrad. Upon entering the second plate 118, the angleP of the beam with respect to the normal of the interface surface willbe defined by sin (P)=sin(G)*n/n_(e)=sin(T). Inside the interface 120,the deviation of a beam from normal to the external plate surfacecorresponds to (G-T) or approximately 0.66 urad. In the case where theinterface 120 thickness is p=100 μm, the approximate local shift of thewave front is p*sin(G-T) or approximately 66 nm. It is further notedthat a reduction below 1 nm may be possible with index matchinginterface material of n˜1.4 and/or reduced interface thickness (e.g., 1μm interface results in local shift of 0.7 nm).

In one embodiment, the complementary surface profiles 117, 119 may becreated by treating the surface of plates 116 and 118 to achieve theindividual height profiles:

${z\; 1\left( {x,y} \right)} = {\frac{d}{2} + {{{\Delta\phi}\left( {x,y} \right)}*\frac{\lambda}{4{\pi\Delta}\; n}}}$${z\; 2\left( {x,y} \right)} = {\frac{d}{2} - {{{\Delta\phi}\left( {x,y} \right)}*\frac{\lambda}{4\pi\;\Delta\; n}}}$

where d/2 is the nominal thickness of each plate, Δφ(x,y) is thedesigned phase delay profile (e.g., π/2 for quarter-wave, π forhalf-wave delay between polarization), λ is the wavelength andΔn=n_(e)−n_(o) is the birefringence of the selected material. In thisregard, the first plate 116 and second plate 118 assembly has a combinedheight of d and a phase delay given by:

${\Delta\phi} = {2\pi*\frac{\Delta\; n}{\lambda}\left\{ {{2{z\left( {x,y} \right)}} - d} \right\}}$

By way of example, in the case of a 2 mm thick MgF₂ (Δn=0.0126) plateilluminated with laser light of wavelength λ=266 nm (e.g., laser lightfrom Nd:Yag laser) the following is found for z1(x,y) and z2(x,y) (inμm):z1(x,y)=1000+Δϕ(x,y)*1.6694z2(x,y)=1000−Δϕ(x,y)*1.6694

In the case of a half-wave plate (i.e., Δφ=π), the deviation of z fromthe nominal d/2 value is 5.244 μm corresponding with 19.71λ. It is notedthat a similar calculation may be applied in the case of crystallinequartz (Δn=0.01078) and other like material.

It is noted herein that the particular profile shapes of the surfaces117, 119 of plates 116, 118 may take on any general form required tocorrect aberrations and/or suppress an undesired signal (discussedfurther herein). The profile shapes may take on periodic or non-periodicform as a function of transverse position across the pupil plane.

FIG. 1D illustrates a flow diagram depicting a method 130 for formingthe dual-sided polarization control device 102, in accordance with oneor more embodiments of the present disclosure.

In a first step 132, a first plate 116 and a second plate 118 areprovided. For example, a flat crystalline plate (e.g., MgF₂ orcrystalline quartz plate) may be cut to form a first plate and a secondplate. For the purposes of the present disclosure, plates having asurface roughness below 0.1λ of the wavelength of illumination areconsidered ‘flat.’ By way of another example, the flat crystalline platemay be cut to form a first plate or second plate having any geometricalshape known in the art, such as, but not limited to, a square, arectangle, a circle and the like.

In a second step 134, the first plate 116 and the second plate 118 aretreated to create complementary surface profiles 117, 119. It is notedherein that the surface profiles 117, 119 may be formed utilizing anysuitable method known in the art. For example, the surface profiles 117,119 may be formed utilizing one or more magnetorheological finishing(MRF) techniques. It is noted herein that MRF techniques may providecontrol of the surface profiles 117, 119 within approximately 1 nm. MRFis generally described by Menapace et al., “Magnetorheological Finishingfor Imprinting Continuous Phase Plate Structure onto Optical Surfaces,”Proceedings of SPIE, Vol. 5273 (2004), which is incorporated herein byreference in the entirety. MRF is also generally described by Tricard etal., “Continuous Phase Plate Polishing Using MagnetorheologicalFinishing,” Proceedings of SPIE, Vol. 7062 (2008), which is incorporatedherein by reference in the entirety. By way of another example, thesurface profiles 117, 119 may be formed utilizing one or more etchingtechniques. By way of another example, the surface profiles 117, 119 maybe formed utilizing one or more ion-beam machining techniques.

In a third step 136, the optical axis 121 of the first plate 116 isaligned orthogonal to the optical axis 123 of the second plate 118. Aspreviously noted herein, the orthogonal alignment of the optical axes121, 123 of the first and second plates 116, 118 provides forbirefringence in the polarization control device 102.

In a fourth step 138, the first plate 116 is positioned proximate to thesecond plate 118. For example, the surface profile 117 of the firstplate 116 and the surface profile 119 of the second plate 118 may bealigned and fit together utilizing a mechanically-coupling means (e.g.,adhesive (e.g., glue, epoxy and the like) or mechanical device (e.g.,clamp)). Further, an index-matching fluid may be used in the interface120 between plates 116, 118.

It is noted herein that the polarization control device 102 of thepresent disclosure may be utilized to correct polarization aberrationsimparted to an illumination beam by the optical elements of an opticalsystem. For example, the polarization control device 102 may be used tocorrect polarization aberrations created by the surfaces/interfaces(e.g., non-normal reflective surfaces) of one or more optical elementsof an optical system.

FIG. 2 illustrates a simplified schematic view of an inspection system200 including one or more parabolic collectors and the polarizationcontrol device 102 used to correct aberrations within the inspectionsystem 200, in accordance with one embodiment of the present disclosure.In one embodiment, inspection system 200 includes an illumination source101 and a sensor 110, as described previously herein with respect tosystem 100. In another embodiment, the system 200 includes apolarization control device 102 and polarizer 104, which have also beengenerally described herein with respect to system 100. It is notedherein that the embodiments and implementations described previouslyherein with respect to the components and implementations of opticalsystem 100 should be interpreted to extend to system 200.

In one embodiment, the inspection system 200 includes one or moreparabolic collectors 204. In another embodiment, the volume within theone or more parabolic collectors 204 of system 200 may be filled with alow-scattering gas, such as, but not limited to, helium.

In one embodiment, the parabolic collector 204 is used to collect lightfrom a sample over substantially large NA, as shown in FIG. 2. It isnoted herein that, at the output, the parabolic collector 204 willdisplay a substantially varying retardance over NA due to non-normalreflections at the collector 204 surface (e.g., mirror). It is furthernoted herein that collector-induced polarization changes may inhibit theability of a given optical system to analyze polarization of collectedlight (e.g., using polarizer 104) or separate between different sourcesbased on polarization distribution over NA.

In one embodiment, the polarization control device 102 is used in the NAplane to correct or reverse collector-induced changes. In this regard,the use of a polarization control device 102 allows for the use of alinear polarizer 104 (also positioned in the NA plane) to analyze apolarization state of collected light. In this regard, the polarizationcontrol device 102 may be constructed to have surface profiles 117, 119suitable for correcting the collector-induced polarization changes. Inthis regard, the polarization control device 102 may includecomplementary individual surface profiles z1(x,y) and z2(x,y) suitablefor continuously altering polarization across the NA of inspectionsystem 200 so as to at least partially correct the collector-inducedpolarization changes of the system 200.

In another embodiment, after light is passed through the polarizationcontrol device 102, the light may then be converted back into imagespace and passed onto sensor 110. For example, in the case of aninspection system 200 including a first parabolic collector 204, lightmay be converted back into image space utilizing a second parabolicobjection 206, as shown in FIG. 2. It is recognized herein that in somesettings the dual-parabolic collector system 200 may replace anellipsoid-shaped collector in some applications.

FIG. 3 illustrates a simplified schematic view of an inspection system300 including a Schwarzchild objective and the polarization controldevice 102 used to correct aberrations within the inspection system 300,in accordance with one embodiment of the present disclosure. In oneembodiment, inspection system 300 includes an illumination source 101and a sensor 110, as described previously herein with respect to system100. In another embodiment, the system may include one or more lenses304. For example, as shown in FIG. 3, the system 300 may include a tubelens. In another embodiment, the system 300 includes a polarizationcontrol device 102 and polarizer 104, which have also been generallydescribed herein with respect to system 100. It is noted herein that theembodiments and implementations described previously herein with respectto the components and implementations of optical system 100 should beinterpreted to extend to system 300.

In one embodiment, the inspection system 300 includes a Schwarzchildobjective 302. In one embodiment, the Schwarzchild objective 302 is usedto collect light from a sample 114 over substantially large NA, as shownin FIG. 3.

It is noted herein that as light passes through the Schwarzchildobjective 302, each light ray undergoes two internal reflections (onebefore and one after the sample), as shown in FIG. 3. In the plane ofincidence of a given light ray, the y-component is in-plane, with thex-component being out of plane. It is further noted that the x-componentdoes not change during propagation through the objection, but they-component is rotated by 180 degrees. As a result, the electric fieldvector is rotated after two reflections by an angle twice that of theazimuthal angle.

As in the case of system 300, the collector-induced polarization changesmay inhibit the ability of a given optical system to analyzepolarization of collected light (e.g., using polarizer 104) or separatebetween different sources based on polarization distribution over NA.

In one embodiment, the polarization control device 102 is used in the NAplane to correct or reverse collector-induced changes. In this regard,the use of a polarization control device 102 again allows for the use ofa linear polarizer 104 (also positioned in the NA plane) to analyze apolarization state of collected light. In this regard, the polarizationcontrol device 102 may be constructed to have surface profiles 117, 119suitable for correcting the polarization changes caused by theSchwarzchild objective 302. In this regard, the polarization controldevice 102 may include complementary individual surface profiles z1(x,y)and z2(x,y) suitable for continuously altering polarization across theNA of inspection system 300 so as to at least partially correct thepolarization changes caused by the Schwarzchild objective 302 of thesystem 300.

In another embodiment, although not shown, the polarization controldevice 102 may be used within an inspection system that implements arefractive-based collector. In this regard, the polarization correctdevice 102 may at least partially correct polarization changes caused bya refractive-based collector on an inspection system.

Referring again to FIG. 1A generally, it is noted herein that thepolarization control device 102 of the present disclosure may beutilized to discriminate between light sources based on the polarizationof the light from the sources. In one embodiment, the polarizationcontrol device 102 in combination with the linear polarizer 104 may beused to suppress one or more unwanted light signals. For example, thepolarization control device 102 in combination with the linear polarizer104 may be used to enhance extinction of ‘haze’ associated with thesurface of the sample 114. As such, the size of one or more signalsassociated with a defect or feature (e.g., particle) may be enhancedrelative to the haze signal associated with the sample surface.

It is noted herein that in certain settings the polarization associatedwith the haze signal becomes elliptical. For example, in settings wherethe azimuthal angle of scattering is larger than +/−45°, the haze signalmay have an elliptical polarization. In this situation, the haze signalcannot be suppressed by a simple linear polarizer. In addition, thedirection of the main axis of the haze ellipse deviates from beingorthogonal to the signal, which remains linear approximately along theradial direction.

In one embodiment, the polarization control device 102 may controlpolarization rotation as a function of position across the pupil planeso as to convert elliptically polarized light associated with haze tolinearly-polarized light. In this regard, the polarization controldevice 102 may serve as a location-specific polarization retarder, whichserves to impart varying levels of polarization rotation to the light asa function of position across the pupil plane 103 so as to producelinearly-polarized light in the same direction. In another embodiment,the polarization control device 102 again includes a first wave plate116 and a second wave plate 118 including surface profiles 117, 119which provide for a continuous variation of imparted polarizationrotation as a function of position (e.g., x, y position) across thepupil plane 103. In another embodiment, the linear polarizer 104 maythen be used to suppress the haze signal associated with the surfacesignal of the sample 114 as it exits the polarization control device 102with linear polarization.

FIG. 1E illustrates a graph 150 including a conceptual depiction of thedirection of polarization for both a desired signal, such as a particle,and the surface background, in accordance with one embodiment of thepresent disclosure. As shown in FIG. 1E, graph 150 shows the directionof polarization for the signal and surface at various x-y phase delays(e.g., phase delays implemented by polarization control device) neededto transform polarization into the required state.

For example, at an azimuthal angle of +135°, a phase delay of 0 appliedto the signal and haze, which are orthogonally polarized at +135°,results in no operation on the polarization state of the signal or thehaze. As such, in this case, the signal and haze are orthogonallylinearly polarized following the phase delay, allowing the linearpolarizer 104 to suppress the haze signal.

By way of another example, at an azimuthal angle of −135°, an x-y phasedelay of πλ/2 applied to the signal and haze, which have orthogonallypolarized states opposite to the +135° case, results in thetransformation of the polarization state of the signal and haze suchthat the polarization states are opposite to the initial polarizationstates. As such, in this case, the signal and haze are orthogonallylinearly polarized following the phase delay, allowing the linearpolarizer 104 to suppress the haze signal.

By way of another example, at an azimuthal angle of +45° C., an x-yphase delay of +π/2(λ4) applied to the signal and haze, which haveopposite circularly polarized states in the +45° C. case, results in therotation of the polarization states of the signal and the haze such thatthe polarization states are converted to orthogonal linear polarizationstates. As such, in this case, the signal and haze are orthogonallylinearly polarized following the phase delay, allowing the linearpolarizer 104 to suppress the haze signal.

By way of another example, at an azimuthal angle of −45° C., an x-yphase delay of −π/2(λ4) applied to the signal and haze, which haveopposite circularly polarized states in the −45° C. case, results in therotation of the polarization states of the signal and the haze such thatthe polarization states are converted to orthogonal linear polarizationstates. As such, in this case, the signal and haze are orthogonallylinearly polarized following the phase delay, allowing the linearpolarizer 104 to suppress the haze signal.

Using the principles outlined above in graph 150, the polarizationcontrol device 102 may be used to transform (or maintain) thepolarization of the signal and/or haze in a spatially continuous manner(using the spatially varying phase delay profiles z1 and/or z2) acrossthe pupil plane 103. Once the light of the signal and/or light of thehaze have been transformed (or maintained) in nominally orthogonallinear states, the linear polarizer 104 may be used to suppress the hazesignal. In this regard, the polarization control device 102 may serve toprovide varying levels of phase retardation as a function positionacross the pupil plane 103 in order to impart the needed retardation toallow a polarizer 104 to suppress the haze (or any other unwantedsignal) relative to the desired signal (e.g., scattering from particle).

FIG. 1F illustrates a conceptual illustration 160 of the conversion ofelliptically polarized background signal, or haze signal, into linearpolarized light, in accordance with one embodiment of the presentinvention. As shown in FIG. 1F, the phase retardation applied to lightpassing through the pupil plane 103 is a continuous function of locationin the pupil. In this regard, once background light across the pupil isconverted into linear polarization (with varying levels of phaseretardation), the light may then easily blocked with a simple linearpolarizer 104.

As shown in FIG. 1F, the path 166 depicts the application of a linearpolarizer at fixed angle with respect to the phase plate. Further, path168 conceptually depicts a phase plate selected to produce optimizedSNR. In this case, background polarization 164 is “shrunk,” or reducedto almost linear polarization, so that the linear polarizer 104 operatesmore effectively on the light. Further, path 170 conceptually depictsthe use of a linear polarizer with location specific preferredpolarization angle.

In one embodiment, the example depicted in FIG. 1F may correspond toillumination with an azimuthal angle of 90°. In this regard, the signal162 (e.g., particle signal) has linear polarization along a firstcoordinate axis, while the haze 164, or background signal, iselliptically polarized and titled from the first axis (e.g., verticalaxis) by approximately 30°. It is noted herein that the descriptionrelated to FIGS. 1E and 1F are not limiting and should be interpretedmerely as illustrative.

While much of the disclosure has focused on a polarization controldevice 102 including two plates 116, 118, it is noted herein that thisis not a limitation of the present invention. FIG. 4 illustrates asimplified schematic view of a single-plate polarization control device402, in accordance with an alternative embodiment of the presentdisclosure. In this regard, it is noted herein that the polarizationcontrol device 402 may be used in a manner similar to the polarizationcontrol device 102 described previously herein. As such, all of thevarious embodiments and implementations of polarization control device102 should be interpreted to extend to the polarization control device402. In one embodiment, the single-plate polarization control device 402includes a surface profile 404, whereby the height z varies as afunction of x-position and y-position. In another embodiment, thepolarization control device 402 includes an optical axis 406, which maybe arranged orthogonal to the illumination direction 408. It is notedherein that the use of a single plate in the control device may beparticularly advantageous in settings where aberration correction (whichmay be created by a single plate) are not required.

FIG. 5A illustrates an optical system 500 equipped with a point spreadfunction control device 502 suitable for modifying point spread function(PSF) of the system 500, in accordance with one embodiment of thepresent disclosure. It is noted herein that the ability to control PSFmay allow for the increase in the amount of energy collected per pixelwithin a given detector, thereby providing increased sensitivity withinan optical system and/or reduced light source (e.g., laser) powerrequirements.

In one embodiment, the system 500 provides for controlling polarizationrotation as a function of position across the pupil plane 503. Inanother embodiment, the system 500 provides for imparting an opticaldelay to the illumination as a function of position across the pupilplane 503. In this regard, the system 500 may modify the PSF of theillumination of the optical system 500 so that the amount ofillumination energy received by one or more pixels matches the size ofthe pixels size of the detector array 512. In this regard, the PSFcontrol device 502 may serve to enhance or maximize the amount ofoptical energy received by one or more pixels 510 of the detector array512.

For example, as shown in FIG. 5B, an untreated illumination beam mayhave a PSF 514 that spreads across multiple pixels 510 of the detectorarray 512. As a result, the amount of optical energy received by asingle pixel is sub-optimal and further requires the reading of multiplepixels for a single detection. In one embodiment, as shown in FIG. 5C,the PSF control device 502 (e.g., phase plate device 535 or opticaldelay device 537) may control polarization rotation or the optical delayin order to control the degree of coherency to form two point spreadfunctions 511, 513 which are incoherent and act to enhance the amount ofenergy received by a given pixel 510.

Referring again to FIG. 5A, in one embodiment, the system 500 includes aset of refractive optics 504 configured to collect light scattered froma feature 507 on sample 508. For example, the refractive optics 504 mayinclude a set of refractive collection elements. For instance, the setof refractive collection elements may include, but is not limited to, acatadioptric objective. In another embodiment, the system 500 includesone or more lenses 506 configured to focus the illumination onto one ormore pixels 510 of a detector array 512. It is noted herein that thedetector array 512 is depicted as being one-dimensional merely fordescriptive convenience and such a depiction should not be interpretedas a limitation on the present invention. For example, the detectorarray 512 may include, but is not limited to, a two-dimensional detectorarray made up of an n×m grid of pixels.

In one embodiment, the PSF control device 502 of system 500 is apolarization control device 535, as shown in FIG. 5D. In one embodiment,the polarization control device 535 includes a pair of phase plates 516and 518, as depicted in FIG. 5D. In an alternative embodiment, thepolarization control device 535 may include a single plate. In oneembodiment, the polarization control device 535 may modify PSF of theoptical system 500 via the modulation of the phase of light as afunction of position across the pupil plane 503. In another embodiment,the polarization control device 535 may modify PSF of the optical system500 via the modulation of the degree of coherence as a function ofposition across the pupil plane 503. It is noted herein that thepolarization control device 535 may be used to improve the amount ofenergy collected by an individual pixel 510 of detector 512.

In this regard, the introduction of controlled polarization rotationwith polarization control device 535 may split light incident on thedetector array 512 into two light distributions of orthogonalpolarizations corresponding with two point spread functions (e.g., 511and 513) in the image plane of the detector array 512. In anotherembodiment, the two point spread functions in the image plane of thedetector array 512 may be added incoherently, which may help minimizediffraction ringing effects while reducing the spread of each PSF,resulting in higher enclosed energy in a given detector pixel 510. Inanother embodiment, the two point spread functions in the image plane ofthe detector array 512 may be added coherently.

In one embodiment, the plates 516 and 518 of the polarization controldevice 535 have complementary surface profiles 517, 519. In anotherembodiment, the first plate 516 of the device 515 has a first opticalaxis 521. Further, the second plate 518 has an optical axis 523, whichis oriented orthogonal to the first optical axis 521. In addition, theoptical axis 521 of the first plate 518 and the optical axis 523 of thesecond plate 518 may be oriented orthogonal to the direction ofillumination propagation through the device 535.

It is noted that the general construction of polarization control device535 and the considerations taken into account when constructing thedevice 535 are similar to that for the polarization control device 102.It is noted, however, that the polarization control device 535 isimplemented within an optical system in order to control PSF, while thepolarization control device 102 is implemented within an optical systemto correct polarization aberrations and/or improve the signal-to-noiseratio in the system. As such, while the construction of devices 102 and535 are similar, it is not necessarily the case (although it ispossible) that the various parameters (e.g., surface profiles, interfacedistance, thickness of component plates, thickness of overall device andthe like) of the devices 102 and 535 will be the same.

In another embodiment, the PSF control device 502 of system 500 includesan optical delay control device 537, as shown in FIG. 5E. In oneembodiment, the optical delay control device 537 includes one or moreplates 530, 532 as depicted in FIG. 5E. It is noted herein that theplates of the optical delay control device 537 of FIG. 5E may be formedin a manner similar to the plates described throughout the presentation.It is noted, however, that the optical delay control device 537 of FIG.5E may include a plate thickness variation on the order of 1-3 mm, whichis larger than a laser pulse length/temporal coherence length.

In one embodiment, the plates 530 and 532 of the optical delay controldevice 537 have complementary surface profiles 531, 533. In anotherembodiment, the first plate 530 of the device 537 has a first opticalaxis 539. Further, the second plate 532 has an optical axis 541, whichis oriented orthogonal to the first optical axis 539. In addition, theoptical axis 539 of the first plate 530 and the optical axis 541 of thesecond plate 518 may be oriented orthogonal to the direction ofillumination propagation through the optical delay control device 537.

The operation of the optical delay control device 537 may be describedas follows. In one embodiment, a subset A of pupil points may haveoptical delay DA (i.e., all points in a subset have some delay), whilesubset B has an optical delay DB. Therefore, pupil points in subset Bhave an optical delay with respect to points in subset A of (DB-DA). Inthe event that (DB-DA) is greater than the coherence length ofilluminated light (or simply larger than the pulse width in the case ofa pulsed light source), then subsets A and B will effectively beincoherent with respect to each other (while points within each subsetmaintain coherence among them). In this case, subsets A and B willcreate independent distributions in an image plane by addingincoherently, and achieve effects similar to that of subsets A and Bhaving orthogonal polarizations. It is noted herein that implementationof an optical delay may be most straightforward from the standpoint ofreduced manufacturing tolerances, while also allowing for multipleincoherent subsets (as opposed to allowing for only two distributions inthe case of the polarization rotation approach). On the other hand, fora typical pulse width (e.g. 20 ps), the optical delay control device 537would have to have thickness variations on the order of a fewmillimeters, which is thicker than that needed for the polarizationrotational plate 535. As such, transitional regions within the opticaldelay control device 537 would display large local surface tilts whencompared to polarization rotation plate 535.

It is recognized herein that the PSF plate (e.g., phase plate 535 oroptical delay plate 537) may be used in conjunction with thepolarization control device 102 described previously herein. Forexample, the polarization control plate 102 may be used to improvesignal-to-noise ratio by, for example, suppressing haze of the sample508 surface relative to the scattering signal from the defect 507.Further, the polarization control device 102 may be used to correctpolarization aberrations within the given optical system. In contrast,the PSF control device 502 (e.g., polarization rotation control device535 or optical delay control device 537) may be used to control the PSFof the illumination and enhance or match the PSF to the one or morepixels 510 of detector 512.

In another embodiment, a given optical system may implement both thepolarization control device 102 and the PSF control device 502 in aneffort to simultaneously improve signal-to-noise, correct aberrationsand enhance PSF matching to the given detector array.

Although particular embodiments of this invention have been illustrated,it is apparent that various modifications and embodiments of theinvention may be made by those skilled in the art without departing fromthe scope and spirit of the foregoing disclosure. Accordingly, the scopeof the invention should be limited only by the claims appended hereto.It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes.

What is claimed is:
 1. An optical system for controlling point spreadfunction comprising: an illumination source configured to illuminate asurface of a sample; a set of collection optics configured to collectillumination from the surface of a sample; a detector including aplurality of pixels; and a point spread function control devicepositioned substantially at a pupil plane of the optical system, thepoint spread function control device including: a first wave platehaving a first surface profile; a second wave plate having a secondsurface profile complementary to the first surface profile, the firstwave plate and the second wave plate positioned so as to substantiallyalign the first surface profile with the second surface profile andmaintain a constant thickness of an assembly of the first wave plate andsecond wave plate, the first wave plate and the second wave plate beingconfigured to control at least one of polarization rotation and degreeof coherence as a continuous function of transverse position across thepupil plane in order modify the point spread function of illumination inorder to enhance an amount of energy received by one or more pixels ofthe detector.
 2. The system of claim 1, wherein the first wave plate andthe second wave plate being configured to control at least one ofpolarization rotation and degree of coherence as a continuous functionof transverse position across the pupil plane in order to splitillumination into a first PSF and at least a second PSF in an imageplane of the detector of illumination in order to enhance an amount ofenergy received by one or more pixels of the detector.
 3. The system ofclaim 2, wherein the detector adds the first PSF and the second PSFincoherently.
 4. The system of claim 2, wherein the detector adds thefirst PSF and the second PSF coherently.
 5. The system of claim 1,wherein the optical axis of the first wave plate is substantiallyorthogonal to the optical axis of the second wave plate.
 6. The systemof claim 1, wherein the set of collection optics includes one or morerefractive optical elements.
 7. The system of claim 6, wherein the oneor more refractive optical elements includes a catadioptric objective.8. The system of claim 1, wherein the optical system is an inspectionsystem.
 9. The system of claim 1, wherein the optical system is a highNA ultraviolet inspection system.
 10. The system of claim 1, wherein atleast one of the first wave plate and the second wave plate is formedfrom an optical crystalline material.
 11. The system of claim 10,wherein the optical crystalline material comprises: at least one ofcrystalline quartz and magnesium fluoride.
 12. The system of claim 1,wherein at least one of the first wave plate and the second wave plateis formed with at least one of magnetorheological finishing technique,an etching technique and an ion-beam machining technique.
 13. The systemof claim 1, further comprising: a polarization control device.
 14. Apoint spread function control system comprising: an illumination sourceconfigured to illuminate a surface of a sample; a set of collectionoptics configured to collect illumination from the surface of a sample;a detector including a plurality of pixels; and a point spread functioncontrol device positioned substantially at a pupil plane of the opticalsystem, the point spread function control device including: a first waveplate having a first surface profile; a second wave plate having asecond surface profile complementary to the first surface profile, thefirst wave plate and the second wave plate positioned so as tosubstantially align the first surface profile with the second surfaceprofile and maintain a constant thickness of an assembly of the firstwave plate and second wave plate, the first wave plate and the secondwave plate being configured to impart an optical delay as a continuousfunction of transverse position across a pupil plane of an opticalsystem in order modify the point spread function of illumination inorder to enhance an amount of energy received by one or more pixels ofthe detector.
 15. The system of claim 14, wherein the first wave plateand the second wave plate are configured to impart an optical delay as acontinuous function of transverse position across a pupil plane of anoptical system in order to split illumination into a first PSF and atleast a second PSF in an image plane of the detector of illumination inorder to enhance an amount of energy received by one or more pixels ofthe detector.
 16. The system of claim 15, wherein the detector adds thefirst PSF and the second PSF incoherently.
 17. The system of claim 15,wherein the detector adds the first PSF and the second PSF coherently.18. The system of claim 14, wherein the optical axis of the first waveplate is substantially orthogonal to the optical axis of the second waveplate.
 19. The system of claim 14, wherein the set of collection opticsincludes one or more refractive optical elements.
 20. The system ofclaim 19, wherein the one or more refractive optical elements includes acatadioptric objective.
 21. The system of claim 14, wherein the opticalsystem is an inspection system.
 22. The system of claim 14, wherein theoptical system is a high NA ultraviolet inspection system.
 23. Thesystem of claim 14, wherein at least one of the first wave plate and thesecond wave plate is formed from an optical crystalline material. 24.The system of claim 23, wherein the optical crystalline materialcomprises: at least one of crystalline quartz and magnesium fluoride.25. The system of claim 14, wherein at least one of the first wave plateand the second wave plate is formed with at least one ofmagnetorheological finishing technique, an etching technique and anion-beam machining technique.
 26. The system of claim 14, furthercomprising: a polarization control device.